This is the ultimate mob grinder it is very effective and easy to make. I'll be making it in creative but it is meant for survival
Step 1: Finding the Perfect Building Space
Find a large flat area.
Step 2: Getting Started
Build a row of twenty blocks.
Step 3: Adding Water
Do the same two blocks over and connect them at the ends. Then fill both ends with water and you should have four blocks in the middle without water.
Step 4: Making a Cross
Now break the two blocks that are not touching water on both sides. Then add a row of eight blocks to it forming a cross. Then fill them with water.
Step 5: Filling in the Empty Spaces
Just follow the pics for this one.
Step 6: Making the Hole of Death
In the four blocks without water dig a really deep hole. It has to be really deep so that the mobs will die when they fall. I think the minimum is 25 blocks but I'm not positive.
Step 7: Getting Back to the Surface
At the bottom of your hole dig into the side of the wall and make a long tunnel. If it isn't long enough you will dig into your grinder. Then dig up putting ladders on the wall as you go. Don't forget to put a door on the big hole that mobs will die in.
Step 8: Making the Room Completely Dark
Add walls and a roof and make sure no light is getting in or mobs won't spawn. It should be completely dark.
Step 9: Enjoy
Just walk down the ladder and go down the tunnel, open th door and TaDa free mob items. Enjoy your unlimited arrows.